Then, the query-document relevance score can be computed using a retrieval function, and top-\(K\)  relevant documents are returned. One of the most popular retrieval functions is BM25 [1]}:
\(\text{BM25}(Q, D) &= \sum _{x\in Q} \text{TF}(x, Q)\text{IDF}(x, \mathcal {D}) \cdot \frac{ \text{TF}(x, D) \cdot (k_1 + 1)}{\text{TF}(x, D) + k_1 \cdot (1-b+b \cdot \frac{|D|}{L})},\) 
